The title compound, 4-vinylbenzeneacetic acid, is a monomer which finds potential as a copolymer with, for example, styrene. The resulting materials may be used as plasticizers, ion exchange resins, and so on. The monomer itself also may be valuable in adhesive formulations. However, applications development has been hindered by the material's limited supply. In particular, there presently is no adequate synthesis utilizing commercially accessible precursors.
The purpose of our invention is to provide a synthetic route to 4-vinylbenzeneacetic acid from 1,4-diethylbenzene, a relatively available material of commerce. The synthesis involves the initial selective oxidation of 1,4-diethylbenzene to 4-ethylacetophenone, oxidatively rearranging the latter to 4-ethylbenzeneacetic acid, selectively monochlorinating the latter acid at the benzylic carbon of the ethyl group, followed by base-catalyzed dehydrochlorination of the resulting 4-(1'-chloroethyl)benzeneacetic acid to 4-vinylbenzeneacetic acid, and recovering the latter.
The initial stage in our preparative route involves the oxidation of 1,4-diethylbenzene with oxygen in the presence of cobalt (II) compounds. Such oxidations typically are performed in air, although oxygen may be substituted therefor, at pressures ranging from atmospheric up to about 100 psig. Oxidation temperatures usually run between about 80.degree. and 140.degree. C., with a narrower range of from about 90.degree. to about 120.degree. C. being preferred.
Among the cobalt (II) compounds which may be used as catalysts for this oxidation are included cobalt phthalocyanine, cobalt salts of carboxylic acids, and cobalt salts of sulfonic acids. Among the carboxylic acids whose salts may be used are included acetylacetic acid, the alkanecarboxylic acids containing from about 8 to about 18 carbon atoms, and naphthalenecarboxylic acid, with the latter being particularly useful. Among the alkanecarboxylic acids whose cobalt salts may be used as catalysts may be mentioned octanoic, nonanoic, decanoic, undecanoic, dodecanoic, tridecanoic, tetradecanoic, pentadecanoic, hexadecanoic, heptadecanoic, and octadecanoic acids. Among the sulfonic acids whose cobalt salts are frequently employed may be mentioned benzenesulfonic acid, toluenesulfonic acid, and methanesulfonic acid, with the toluenesulfonate salt finding broadest use.
In the aforementioned oxidation one methylene group of 1,4-diethylbenzene is transformed into a carbonyl group to afford as the product 4-ethylacetophenone. This latter compound is then oxidatively rearranged by the Kindler modification of the Willgerodt reaction. In this reaction, the ketone is heated with sulfur and an amine, the resulting intermediate is hydrolyzed with base, and after acidification there is obtained the resulting 4-ethylbenzeneacetic acid.
Generally both sulfur and the amine are used in approximately the same molar proportion, with from about 1 to about 8, preferably from about 2 to about 4, molar proportions of each, relative to 4-ethylacetophenone. Among the amines which may be employed morpholine is especially advantageous, as are alkyl-substituted pyridines, e.g., the lutidines and collidines, and the picolines. However, it is to be understood that a wide variety of other tertiary amines may be employed, although not necessarily with equivalent results. The reaction is conducted at a temperature from about 100.degree. to about 150.degree. C., more particularly between about 115.degree. and 135.degree. C. It has been found that removal of water during the heating period is beneficial. Reaction times typically are from about 1 to about 5 hours, depending upon the temperature employed.
After reaction with sulfur and the tertiary amine is complete the resulting intermediate is hydrolyzed with alkali. Aqueous solutions of the alkali metal hydroxides are most conveniently employed, although other alkaline materials, such as the alkali metal carbonates, may be used without prejudice. Typically, a large excess of a relatively strong (10-30%) solution of alkali is employed with hydrolysis being conducted at about 100.degree. C. for a time between about 5 and about 15 hours. After base hydrolysis the reaction mixture is acidified, generally with any mineral acid, to liberate the free acid and unreacted sulfur. Sulfur is readily removed from the organic acid mixture by treatment with base, for example, sodium bicarbonate, which dissolves the organic acid selectively.
The next step in the preparation is photochlorination of 4-ethylbenzeneacetic acid using ultraviolet irradiation with between about 1 and 2 molar proportions of chlorine. Chlorination is continued to about 50% conversion in order to maximize selectivity of monochlorination. Typically, photochlorination is conducted in an ultraviolet transparent solvent, itself not chlorinated under reaction conditions, with chlorinated alkanes, such as carbon tetrachloride, chloroform, methylene chloride, hexachloroethane, and so on, being commonly used. Photochlorination may be conducted at a temperature between about 20.degree. and about 80.degree. C., even more usually between about 35.degree. and about 65.degree. C.
The last stage in our preparative sequence involves dehydrochlorination of 4-(1'-chloroethyl)benzeneacetic acid with an alkali metal hydroxide or alkoxide. Among the hydroxides sodium hydroxide and potassium hydroxide are preferred. Among the alkoxides the sodium, potassium, and lithium salts of primary and secondary alcohols containing up to about 6 carbons are most usually employed. Examples include such alkoxides as sodium methoxide, potassium methoxide, lithium methoxide, potassium ethoxide, sodium propoxide, lithium butoxide, sodium pentoxide, and sodium hexoxide. Solutions of the base may vary between about 5% and about 20% and the reaction is effected between about 25.degree. and about 80.degree. C. The title compound, 4-vinylbenzeneacetic acid, then is isolated from the reaction mixture as by acidification followed by extraction.